The present invention relates to a method and a device that reduces the influence of the mechanical environment on the electrical properties of travelling wave tubes.
The Travelling Wave Tube--TWT--is a wideband amplification tube for signals within the microwave range. It is used, for example, in certain types of radio links and in radar stations. In transmission-coherent doppler radar systems it is usual to utilize grid pulsed TWTs as transmission pulse amplifiers. In these systems the transmitted spectral purity of the signal is of great importance for, among other things, the ability of the system to suppress clutter signals. A TWT is however influenced by the mechanical vibration environment and in particularly exposed environments, as in aeroplanes or target tracking missiles, influence of vibrations can seriously impair the performance characteristics of the system.
The source of the problems is when the TWT is subjected to mechanical vibrations, movements between the electrodes of the electron gun will occur. These movements result in the current beam through the tube receiving a modulation at the vibration frequency. For frequencies where there exist mechanical oscillatory resonances in the electron gun, the current modulation becomes considerable.
The current modulation caused by the vibration leads to the microwave signal amplified in the TWT obtaining an amplitude and phase modulation that is proportional to the current modulation. The microwave signal will therefore obtain side bands located at a distance equal to the vibratory frequency on both sides of the carrier frequency. In radar systems this gives rise to deteriorated clutter suppresion for target speeds that, from a doppler point of view, correspond to the vibration frequencies. The usual way to get around the problems of current modulation is to isolate the tube, or the complete transmitter, or even the complete radar station, from vibrations. Such measures are however complicated and voluminous and do not always give a satisfactory result. In certain applications, such as target tracking missiles, it is, from a space point of view, hardly possible with such measures.
In certain applications it can be possible to avoid these difficulties by replacing the TWTs with other components that are less sensitive to vibrations with regard to spectral purity. Injection locked magnetrons or cross field amplifiers can, by way of example, be used. These components, on the other hand, have inferior high frequency characteristics with respect to bandwidth, pulse formation and noise and due to this, other system performance characteristics deteriorate as well.